Overview of laser, timing, and synchronization issues

John Corlett, July 2004

Overview of laser, timing, and

synchronization issues

John Corlett, Larry Doolittle, Bill Fawley, Steven Lidia, Bob Schoenlein, John Staples, Russell

Wilcox, Sasha Zholents

LBNL


• Diffraction and spectroscopy • Nuclear positions and electronic, chemical or structural

probes

Scientific goal - application of ultrafast x-ray sources to study dynamics with high-

resolution

diffraction angle

time delay

time delay

x-ray probe

visible pump

detector

Time-resolved x-ray diffraction

Time-resolved EXAFS

NEXAFSdelay

x-ray probe

visible pump

r

energy

time

Kedge

abso

rptio

n

f(r)

Plus photoelectron spectroscopy, photoemission microscopy, etc

• Access new science in the time-domain x-ray regime


• Ultrafast laser pulse “pumps” a process in the sample• Ultrafast x-ray pulse “probes” the sample after time ∆t

• Ultrafast lasers an integral part of the process• X-rays produced by radiation in an electron accelerator

Pump-probe experiment concept

Laser excitation pulse

X-ray probe pulse

∆t

ion or e- detector

-detector

sample


• Both laser and x-ray pulses should be stable in temporal and spatial distributions

• Parameters and quality of x-ray pulse determined by the electron beam

• Accelerator parameters • Synchronization between laser and x-ray pulses, ∆t, should ideally

be known and controllable - to the level of the pulse duration itself ~ 10 fs

Pump-probe experiment concept

Laser excitation pulse

X-ray probe pulse

∆t

ion or e- detector

-detector

sample


Many projects around the world are addressing the need for ultrafast x-rays, in different ways

• LCLS: linac SASE (construction)• BNL DUV FEL: linac HGHG (operational)• DESY TTF-II: linac SASE (construction)• SPPS: linac spontaneous emission from short bunches

(operational)

• ALFF: linac SASE • BESSY FEL: linac HGHG• European X-ray FEL: linac SASE• Daresbury 4GLS: ERL HGHG + spontaneous• LUX: recirculating linac HGHG + spontaneous• Cornell ERL: ERL spontaneous• MIT-Bates X-ray FEL: linac HGHG + SASE• Arc-en-Ciel: recirculating linac / ERL HGHG + SASE• FERMI@Elettra: linac HGHG• BNL PERL: ERL spontaneous


What are the difficulties in achieving x-ray beam quality?

• X-rays are produced by electrons emitting synchrotron radiation in an accelerator

• The electron beams are manipulated by rf and magnetic systems• The x-ray beam quality is limited by the electron beam quality in

many ways– Electron bunch charge, energy, emittance, energy spread, bunch

length, position, … • At the radiator!

• Production of high-brightness bunches is tough enough– Emission process, space charge, rf focusing, ….

• Then we must accelerate and otherwise manipulate the bunches before they reach the radiating insertion device

• Many opportunities to degrade the electron bunch– Space charge, rf focussing, emittance compensation, CSR, geometric

wakefields, rf field curvature, resistive wall wakefields, optics aberrations, optics errors, alignment, rf phase errors, rf amplitude errors, …


Synchronization

• In addition to the electron bunch properties, the need for synchronization of the x-ray pulse to a reference signal - the pump - is required for many experiments– Time between pump signal and probe x-ray pulse

• Predictable or measurable• Stable to ~ pump & probe pulse durations

• This presents additional demands on the accelerator, instrumentation, and diagnostics systems

• Various techniques may be employed to enhance synchronization– Slit spoiler for SASE – Seeding

• HGHG• ESASE (Enhanced SASE)

– e- bunch manipulation & x-ray compression– Measurement of relative x-ray - pump laser timing

• Electro-optic sampling of electron bunch fields• Time-resolved detection of x-ray and laser pulses at the sample


The roles of lasers, timing,and synchronization in an ultrafast x-ray

facility• Laser systems

– Generate the high-brightness electron beam in an rf photocathode gun– Produce the pump signals at the beamline endstations

• Timing system– Provides reference signals to trigger (pulsed) accelerator systems – Provides reference waveforms to synchronize rf systems– Provides reference waveforms to synchronize endstation lasers

• Synchronization– To control and determine the timing of the x-ray pulse with respect to

a pump pulse – Requires stable systems in the x-ray facility, connected by a “stable”

timing system including stable timing distribution systems• The timing system only has to be “stable” enough for all of the components

connected to it to follow it’s timing jitter (to the required level)• Phase noise ˛ timing jitter

• The majority of the timing jitter must be within the bandwidth of the accelerator & laser systems such that they can follow

– Local feedback around rf & laser systems– Lock to timing system master oscillator

0

f

f

rms f2

df)f(L2t

2

1

π=Δ

∫


0

f

f

rms f2

df)f(L2t

2

1

π=Δ

∫

Phase noise and timing jitter


Some space and time parameters for a conceptual ultrafast x-ray facility

rf photocathode gun Linac Undulators

Bend magnets / compressor

End stations

• 10 fs ≈ 3 µm at c• Thermal expansion for ∆T = 0.1°C in Cu over 100 m

≈ 170 µm or 570 fs• Similar magnitude effect from refractive index change in optical fiber

Length scale ~ 100’s mTime scale ~ µsEquivalent bandwidth ~ 100’s kHz


Some rf systems parameters for a conceptual ultrafast x-ray facility

rf photocathode gun Linac Undulators

Bend magnets / compressor

End stations

• 10 fs ≈ 5x10-3 °rf phase L-band• Cavity filling time (Q=104) ≈ 2 µs

• Bandwidth ~ 100 kHz• Cavity filling time (Q=107) ≈ 2 ms

• Bandwidth ~ 100 Hz • 10 fs ≈ 1x10-2 °rf phase S-band• Cavity filling time (Q=104) ≈ 1 µs

• Bandwidth ~ 300 kHz


Synchronize rf systems to a master oscillator

• Control phase and amplitude of the rf fields experienced by the electron beam

• The master oscillator must have a phase noise spectrum such that the majority of the timing jitter is accumulated within the bandwidth of the rf systems

• Local feedback ensures that the rf systems follow jitter in the master oscillator

• Noise sources• Microphonics• Thermal drift• Electronic noise• Digital word

length


Choice of master oscillator

• rf crystal oscillator has low noise close to carrier• Laser has low noise above ~ 1 kHz

• Mode locked laser locked to good crystal oscillator provides a suitable master oscillator • Active mode-lock cannot respond rapidly to perturbations


Although there are very good low-noise sapphire loaded cavity oscillators

http://www.psi.com.au/pdfs/PSI_SLCO.pdf


Phase noise spectrum requirement

• Master oscillator phase noise within bandwidth of feedback systems can be corrected

• Residual uncontrolled phase noise plus noise outside feedback systems bandwidth results in timing jitter and synchronization limit


Laser synchronization

• two independent psec Mira 900-P (Coherent) lasers• PLLs at 80 MHz (n=1) and 14 GHz (n=175)

D.J. Jones et al., Rev. Sci. Instruments, 73, 2843 (2002).

time (sec)

• Sub-femtosecond timing jitter has been demonstrated between two mode-locked Ti:sapphire lasers• Limit is electronic noise (under favorable conditions)


Sophisticated laser systems are an integral component of an FEL facility

Multiple beamline

endstation lasersPhotocathode

laser

FEL seed lasers

Laser oscillator

Spatial profiling

Amplitude

control

Amplifier

Pulse shaping

Multiply

Laser oscillator Amplifier & conditioning





Lasers may be synchronized to a common master oscillator

Photocathode laser

FEL seed lasers

Laser oscillator

Spatial profiling

Amplitude

control

Amplifier

Pulse shaping

Multiply





Multiple beamline

endstation lasers

Laser master oscillator


rf systems need to be synchronized to a common master oscillator

Photocathode laser

FEL seed lasers

Laser oscillator

Spatial profiling

Amplitude

control

Amplifier

Pulse shaping

Multiply





Multiple beamline

endstation lasers


~


rf signals for the accelerator may also be derived from the laser master oscillator

Photocathode laser

FEL seed lasers

Accelerator RF signals

Laser oscillator

Spatial profiling

Amplitude

control

Amplifier

Pulse shaping

Multiply





Multiple beamline

endstation lasers

~

~ ~ ~ ~ ~ ~



rf photocathode gun


rf photocathode laser

250 300 350 400 4500

20406080

100120140

150

200

250

300

350

0 20 40 60 80100120140160

Vertical lineout

Horizontal lineout

-3 -2 -1 0 1 2 3 4 50

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Time (ps)

UV

Pow

er (

V)

File: cc120605, RMS length = 1.07 ps

UV pulse time profile

• UV pulse on cathode

• W. S. Graves, MIT-Bates (DUV FEL, Brookhaven)


rf photocathode laser

• W. S. Graves, MIT-Bates (DUV FEL, Brookhaven)

50 100 150 200 250 300 350 400

20406080

100120140160180200

HeadTail

0 0.2 0.4 0.6 0.8 1 1.2 1.40

100

200

300

400

500

600

700

Time (ps)

Cur

rent

(A

)

File: phiminusg, FWHM = 0.474 ps

-3 -2 -1 0 1 2 3 4 50

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Time (ps)

UV

Pow

er (

V)

File: cc120605, RMS length = 1.07 ps

UV pulse time profile

Bunch production

, acceleratio

n, and compressi

on

• UV pulse on cathode– Non-uniformity exacerbates space-charge effects– Temporal non-uniformity induces micro-bunching


Laser pulse shaping influences the emitted electron bunch

gratingstretcher

Ti:sapphireRegenerative

Amplifier

Q-switchedNd:YAG (2)

gratingcompressor

Pulse Shaper

Ti:sapphireOscillator

100 fs, 2 nJ<0.5 ps jitter

RF from master

oscillator

23>1 mJ, 800 nm, 10 kHz

PockelsCell

polarizer

photo-switch

spectral filter (computer controlled) - spatial light modulator - acousto-optic modulator

Pulse Shaper (A.M. Weiner)

Dazzler - FastLite Inc.acousto-optic dispersive filter

(P. Tournois et al.)

acoustic wave (computer programmable) - spectral amplitude - temporal phase

TeO2 crystal

Pulse Amplitude StabilizerPatent:: LLNL (R. Wilcox)

Deformable mirror


Laser-driven photocathode - one of the many laser systems

H. Tomizawa, JASRI

R. Cross, J. Crane, LLNL

• Need high reliability– Integrated systems– “hot spare” system attractive– Develop techniques for pulse shaping


rf gun phase and amplitude

• LUX rf gun concept as an example• Assume 5% of bunch length (1 psec) jitter• Primary drivers are launch phase, cell 1 gradient and bunch

charge (laser intensity)• Assumed uncorrelated disturbances: three most significant

parameter tolerances are (rms values):– Launch phase: 0.43 degree– Cell 1 gradient: 1.4% variation– Bunch charge: 36% variation

• Laser–1 µJ, 35 ps, 10 kHz, 266 nm–Spatial and temporal control to provide low-emittance electron bunches

• Cathode–Cs2Te

• RF field–64 MVm-1 at cathode


Nominal LCLS Linac Parameters for 1.5-Å FEL

Single bunch, 1-nC charge, 1.2-Single bunch, 1-nC charge, 1.2-m m sliceslice emittance, 120-Hz repetition rate… emittance, 120-Hz repetition rate…

(RF phase: (RF phase: rfrf = 0 is at accelerating crest)= 0 is at accelerating crest)

SLAC linac tunnelSLAC linac tunnel research yardresearch yard

Linac-0Linac-0L L =6 m=6 m

Linac-1Linac-1L L 9 m9 m

rf rf 25°25°

Linac-2Linac-2L L 330 m330 mrf rf 41°41°

Linac-3Linac-3L L 550 m550 mrf rf 10°10°

BC-1BC-1L L 6 m6 m

RR5656 39 mm39 mm

BC-2BC-2L L 22 m22 m

RR5656 25 mm25 mm LTULTUL L =275 m=275 mRR56 56 0 0

DL-1DL-1L L 12 m12 mRR56 56 0 0

undulatorundulatorL L =130 m=130 m

6 MeV6 MeVz z 0.83 mm 0.83 mm 0.05 %0.05 %

135 MeV135 MeVz z 0.83 mm 0.83 mm 0.10 %0.10 %

250 MeV250 MeVz z 0.19 mm 0.19 mm 1.6 %1.6 %

4.54 GeV4.54 GeVz z 0.022 mm 0.022 mm 0.71 %0.71 %

14.1 GeV14.1 GeVz z 0.022 mm 0.022 mm 0.01 %0.01 %

...existing linac...existing linac

newnew

rfrfgungun

21-1b21-1b21-1d21-1d XX

Linac-Linac-XXL L =0.6 m=0.6 mrfrf= =

21-3b21-3b24-6d24-6d

25-1a25-1a30-8c30-8c

P. Emma, SLAC


X-bandX-band XX--

Jitter tolerance budget Jitter tolerance budget for for LCLSLCLS based on the based on the many sensitivitiesmany sensitivities

Jitter tolerance budget Jitter tolerance budget for for LCLSLCLS based on the based on the many sensitivitiesmany sensitivities

rmsrms ΔΔtt-jitter-jitter = = 109 fs109 fszz jitter jitter == 14 % rms 14 % rms

……and test the budget with jitter simulationsand test the budget with jitter simulations

Jitter Tolerance Levels in the Jitter Tolerance Levels in the LCLSLCLS

Jitter simulation, tracking 10Jitter simulation, tracking 1055 particles 2000 times, where particles 2000 times, where each run is randomized in its 12 each run is randomized in its 12 main rf-parameters according to main rf-parameters according to the tolerance budgetthe tolerance budget

Jitter simulation, tracking 10Jitter simulation, tracking 1055 particles 2000 times, where particles 2000 times, where each run is randomized in its 12 each run is randomized in its 12 main rf-parameters according to main rf-parameters according to the tolerance budgetthe tolerance budget

LCLSLCLS

• P. Emma, SLAC


SASE FEL output

• The SASE FEL process arises from noise

http://www.roma1.infn.it/exp/xfel/SaseXfelPrinciples/Sasexfelprinciples.pdf

Radiation intensity build-up along undulator

Half way along undulator

Saturation


2.6

mm

rm

s2.

6 m

m r

ms

0.1 mm (300 fs) rms0.1 mm (300 fs) rms

Easy access to Easy access to timetime coordinate coordinate along bunchalong bunch

LCLSLCLS BC2 bunch compressor chicane BC2 bunch compressor chicane (similar in other machines)(similar in other machines)

xx , h

oriz

onta

l pos

. (m

m)

, ho

rizon

tal p

os.

(mm

)

zz, longitudinal position (mm), longitudinal position (mm)

50 50 mm

Slit spoiler defines radiating region of bunch

Paul Emma, SLAC


Add thin slotted foil in center of chicaneAdd thin slotted foil in center of chicane

1-1-m emittancem emittance



AFTER FOILAFTER FOIL

BEFORE FOILBEFORE FOIL

Paul Emma, SLAC


Timing determination from Electro Optic sampling -developing techniques at the SPPS

Er

Principle oftemporal-spatial correlation

Line image camera

polarizer

analyzer

EO xtal

seconds, 300 pulses: z = 530 fs ± 56 fs rms Δt = 300 fs rmsseconds, 300 pulses: z = 530 fs ± 56 fs rms Δt = 300 fs rms

single pulse

A. Cavalieri

centroidwidth


ESASE - Enhanced Self-Amplified Spontaneous Emission

BunchingAcceleration SASE

70 as70 as

Modulation

A. Zholents - Wednesday


P0 = 235 GWWith a duty factor = 40,

Paverage~ 6 GW

70 as

• Each micro-pulse is temporally coherent and Fourier transform limited

• Carrier phase is random from micro-pulse to micro-pulse • Pulse train is synchronized to the modulating laser

L=800 nm

x-ray macropulse

Enhanced Self-Amplified Spontaneous Emission


Dispersive section strongly increases bunching at fundamental

wavelength and at higher harmonics

In a downstream undulator resonant at 0/n, bunched beam strongly

radiates at harmonic via coherent spontaneous emission

nπ-nπphase

energ

y

-π π

Input Outpute-beam phase space:

Energy-modulate e-beam in undulator via FEL resonance with coherent input radiation

Harmonic generation scheme -coherent source of soft x-rays

L.-H. Yu et al, “High-Gain Harmonic-Generation Free-Electron Laser”, Science 289 932-934 (2000)L.H. Yu et al., "First Ultraviolet High Gain Harmonic-Generation Free Electron Laser", Phys. Rev. Let. Vol 91, No. 7, (2003)

modulator radiatorbunching chicane

laser pulse

e- bunchDeveloped and demonstrated by L.-H. Yu et al, BNL


seed laser pulse modulator 3rd - 5th harmonic radiator

modulator 3rd - 5th harmonic radiator

Cascaded harmonic generation scheme

Delay bunch in micro-orbit-bump (~50 m)

Low electron pulse

Unperturbed electrons

seed laser pulse

tail head

radiator radiatormodulatormodulator

disrupted region


User has control of the FEL x-ray output properties through the seed

laser • OPA provides controlled optical seed for the free electron laser

• Wavelength tunable – 190-250 nm

• Pulse duration variable– 10-200 fs

• Pulse energy– 10-25 µJ

• Pulse repetition rate– 10 kHz

• Endstation lasers seeded by or synchronized to Ti:sapphire oscillator

– Tight synchronization <20 fs

Ti:sapphireOscillator

<100 fs, 2 nJ<50 fs jitter

gratingstretcher

Ti:sapphireRegenerative

Amplifier

Q-switchedNd:YAG (2)

gratingcompressor

RF derived from optical from master

oscillator

~1 mJ, 800 nm, 10 kHz

Optical Parametric Amplifier

>10% conv. efficiency

e-beam

laser seed pulse

undulator undulator

undulator harmonic

n undulator stagesx-ray

Endstation synch.


Gas jet

Seeding with XUV from high harmonics in a gas jet (HHG)

• Coherent EUV generated up to ~ 550 eV– R. Bartels et al, Science 297, 376 (2002), Nature 406, 164

(2000)

H. Kapteyn, JILA/Uni. Colorado/NIST

E field

Harmonic emission

302520151050Time(fs)

I. Christov et al, PRL 78, 1251, (1997)

45 39 29 25 17Harmonic order

67.5eV 25.5eV

J. Zhou et al, PRL 76(5), 752-755 (1996)


Seeding multiple cascades from a single electron bunch allows 10 kHz operation in

LUX concept

• Optical pulses overlap different part of bunch for each beamline• Timing jitter influences number of cascades that can be served

by a single bunch• CSR effects in the arcs introduce ~ few fs jitter for ~ few %

charge variation

e-beam

FEL optical pulses


800 nm

spectral broadening and

pulse compression

e-beam

harmonic-cascade FEL

two period wiggler tuned for FEL

interaction at 800 nm

2 nm light from FEL

2 nm modulator chicane-buncher

1 nm radiator

dump

endstation

1 nm coherent radiation e-beam

endstation

time delay chicane

Laser-manipulation produces attosecond x-ray pulses in harmonic

cascade FEL

e-beam

A. Zholents, W. Fawley, “Proposal for Intense Attosecond Radiation from an X-Ray Free-Electron Laser”, Phys. Rev. Lett. 92, 224801 (2004)


Ultrafast x-ray pulses by electron bunch manipulation and x-ray compression

2 ps

~ 50 fs

RF deflecting cavity

Electron trajectory

in 2 ps bunch


Master Oscillator Laser

Δt Δt

electron bunch

laser pulse

x-rays

RF

crab cavity3.9 GHz

x-ray pulse compressionasymmetric Bragg x-tals

Δy

Δt

ΔyLow-noise

Amp3.9 GHz

• Synchronization dependent on phase of deflecting cavity• Phase lock to master oscillator

• Fast feedback systems around scrf• Extend frequency response of the system

Synchronize deflecting cavities and pump laser for hard x-ray production


Typical end station concept

Precisely timed laser and linac x-ray pulses

Linac x-ray pulse

Laser master

oscillator pulse

End station

Pulse diagnosticsLaser and delay lines

~ 10 m

Modelocked

Oscillator

• Active laser synchronization– Independent oscillators at each

endstation– Complete independence of endstation

lasers– Wavelength, pulse duration,

timing, repetition rate etc.


Ti:sapphire

Oscillator<100 fs, 2

nJ<50 fs jitter

gratingstretcher

Ti:sapphire

Regenerative

Amplifier

Q-switchedNd:YAG (2w)

gratingcompressor

>1 mJ, 800 nm, 10 kHz

Optical Parametri

c Amplifier

Beamline endstation lasers

PCPC

/4

typical regenerative amplifier

~20 passes ΔL=1 µm (Δt=66 fs)

• interferometric stabilization• cross-correlate with oscillator (compress first)• temperature stabilize (Zerodur or super-invar)

chirped-pulse amplification

RF derived from optical

master oscillator


All-optical timing system to achieve synchronization between laser pump and x-

ray probe• Laser-based timing system• Stabilized fiber distribution system• Interconnected laser systems

• Active synchronization • Passive seeding• rf signal generation 20–50 fs synchronization

Photo InjectorLaser

RF cavity

Master OscillatorLaser

FELSeed Laser

MultipleBeamline Endstation

Lasers

FELSeed Laser

Linac RF

Optical fiber distribution network


cw reference laserinterferometer

L~100 m

Path Length ControlΔL= 2 m

Δt= 7 fs

Agilent 5501B210-9 one hour (Δ210-8 lifetime

Beamline 1

Beamline 2

fiber-based system EDFA(fiber amp)

PZT controlpath length

EDFA(fiber amp)

Master Oscillator

Timing distribution

positiondetector

positiondetector

Master Oscillator


Beamline 2

Beamline 1

free-space system (in vacuum)



L~100 m

Path Length ControlΔL= 2 m

Δt= 7 fs

Agilent 5501B210-9 one hour (Δ210-8 lifetime

Beamline 1

Beamline 2

fiber-based system EDFA(fiber amp)

PZT controlpath length

EDFA(fiber amp)

Master Oscillator

Timing distribution - fiber systems developed fro distribution of frequency

standards

2

4

6

80.1

2

4

6

81

2

4

6

Jitter spectral density (fs / Hz

1/2

)

101

102

103

104

105

106

107

Fourier Frequency (Hz)

1

2

3

4

56

10

2

3

4

56

100

Integrated jitter (fs)

4 km DSF in lab, unstabilized 4 km DSF in lab, stabilized

Integrated jitter

Mixer/amplifier noise floor

D. Jones, UCB/JILA


Modelocked fiber laser oscillatorrf stabilized

28 dB AMP

RF Clock1.3/n GHz

Amplifier

LPF

error signal

17 dBm mixer

Modelocked Laser1.3 GHz

Trep

BPF 1.3 GHzf1/Trep

Modelocked Fiber Laser Oscillator – RF Stabilized

• Phase-lock all lasers to master oscillator• Derive rf signals from laser oscillator• Fast feedback to provide local control of accelerator rf

systems Synchronization 10’s fs


SummaryLasers, timing,and synchronization

• Laser systems under development at many institutions • Applications for improved light-source operations

• Photocathode laser, timing system master oscillator, FEL seed laser, endstation pump laser

• Manipulation of e- beam by laser has great potential• HHG power increasing, wavelength decreasing

• Ultra-stable timing systems with optical fiber distribution systems under development • Application of techniques to accelerator environments

and requirements is to be demonstrated • 10’s fs synchronization seems achievable

Documents

Overview of laser, timing, and synchronization issues